Micron X-ray computed tomography based on micro-particle-induced X-ray emission

Abstract

Particle-induced X-ray emission (PIXE) is a phenomenon of atomic inner shell ionization and characteristic X-ray emission due to ion-atomic collisions. The intensity of continuous X-rays in the PIXE energy spectrum is much lower than that of characteristic X-rays. Therefore, PIXE can be used as a semi-monochromatic X-ray source. Furthermore, PIXE produced by a heavily charged particle beam with a diameter of several micrometers (micro-PIXE) can be used as a monochromatic X-ray point source for Xray computed tomography (CT) to investigate internal structures of objects <1 mm. We developed micron X-ray CT based on micro-PIXE with a spatial resolution of about 4 μm. Because the photoelectric effect cross-section is proportional to the fifth power of the atomic number, the distributions of small amounts of heavy elements can be investigated using this CT technique, and the element can be identified using its absorption edge. We applied this CT to observe the internal structure of hair, a head of an ant, and Drosophila. We were able to identify the medulla configuration and cortex of the hair, and the mandibular glands, pharynx, and brain in the ant head. We confirmed a high Mn concentration in the mandibular glands. We used a contrast agent to visualize the internal organs of Drosophila. Furthermore, we applied this CT to research clay particles contaminated by the Fukushima Dai-ichi nuclear accident and confirmed that cesium atoms were distributed on the surfaces of clay particles.